With the electret condenser microphone noted above, the diaphragm electrode is vibrated by inputted sounds to vary the capacitance of the capacitor section, as a result of which the converter circuit outputs electric signals in response to the variations of the capacitance of the capacitor section. Thus, the electret condenser microphone serves for outputting electric signals in response to inputted sounds.
A conventional electret condenser microphone will be described with reference to FIGS. 13 and 14. FIG. 13 is a cross section and FIG. 14 is a vertical section of the microphone.
The conventional electret condenser microphone comprises a capacitor section 104 including a fixed electrode 103 having a diaphragm 100, a spacer 101 and an electret member 102. The conventional electret condenser microphone further comprises a metal casing section 108 accommodating the capacitor section 104, and a circuit board 106 including a converter circuit 105 for converting variations of capacitance of the capacitor section 104 caused by vibrations of the diaphragm electrode 100 to electric signals for output. Also, the conventional electret condenser microphone comprises a ring-shaped conducting section 107 arranged within the casing section 108 for allowing the capacitor section 104 to be conductive with the circuit board 106, and an insulating tubular member 109 enclosing the capacitor section 104, circuit board 106 and conducting section 107 (see Patent Document 1, for example).
As shown in FIG. 13, according to this electret condenser microphone, the casing section 108 has a circular shape as viewed from top and the capacitor section 104 also has a circular shape as viewed from top.
According to another example of the conventional devices, the casing section has a rectangular shape as viewed from top and the capacitor section also has a rectangular shape as viewed from top (see Patent Document 2, for example).
Patent Document 1:                Japanese Patent Unexamined Publication No. 2001-8293        
Patent Document 2:                Japanese Patent Unexamined Publication No. 2003-78997        